The invention relates to a level control arrangement for controlling the level of a frequencydivision multiplex signal.
Level control arrangements are inter alia used in carrier telephony systems. In such a system a plurality of telephone signals are transmitted, in a FDM-format, from a transmitter to a receiver via a transmission medium, frequently a transmission cable.
As a cable attenuates the signals propagating through it, the level of the FDM signal decreases, namely by a certain amount per kilometer. This means that when the transmitter and receiver are situated at a very great distance from one another, for example, some thousands of kilometers, the level of the FDM-signal will be much too low on receipt by the receiver. To obviate this, repeater stations are arranged in the transmission cable at regular distances to amplify the level of the FDM-signal, entering the repeater, to a predetermined value. The cable attenuation is not a constant quantity because it depends on a number of external factors. In order to determine the amount of change of the signal level occurring between two consecutive repeater stations, a pilot signal is transmitted with the FDM-signal with a certain fixed frequency and with a level which is accurately known at the moment this pilot signal is added to the FDM-signal.
If, now more particularly, the FDM-signal is formed by a so-called primary FDM group, then this FDM-signal comprises 12 telephone signals, each having a bandwidth of 4 kHz and this group of 12 telephone signals is then located in the frequency band from 60 to 108 kHz inclusive. For such a primary FDM group the above mentioned pilot signal has a frequency of 84.140 kHz.
In order to amplify in such a repeater station the level of the incoming FDM signal to the desired level, it is known to provide the level control arrangement with a pilot filter for selecting the pilot signal, to compare the level of the pilot signal thus obtained with a fixed reference level, to generate a signal which is a measure of the difference between the two levels and to set with last-mentioned signal the gain factor of an amplifier having an adjustable gain factor. In case the FDM signal consists of a primary FDM group, said pilot filter must be formed by means of a bandpass filter having a bandwidth of approximately 20 Hz and an intermediate frequency of approximately 84.140 kHz. It is, however, very difficult and, consequently, very expensive to produce a filter which satisfies these requirements.
To diminish the difficulties in making the pilot filter it is known from French Pat. No. 2,287,811, which corresponds to U.S. Pat. No. 4,009,350, to use a level control arrangement comprising a transposition circuit to which the FDM signal is applied and by means of which this signal is transposed to a lower frequency band, in such a way that, for example, the pilot signal of the primary FDM group is converted into a 140 Hz signal. The transposed FDM signal thus obtained is passed on to a low-pass filter.
The output signal of said low-pass filter usually comprises, in addition to the above-mentioned, desired 140 Hz signal, a plurality of signalling signals which are also comprised in a FDM signal and whose frequencies are near to those of this pilot signal. In the case of the so-called d.c. signalling a very strong direct current component occurs in the output signal of the low-pass filter as well as signals having a frequency of 10 Hz and multiples thereof. It should be noted that the amplitude of these signals decreases to a very great extent at higher frequencies.
In order to select the desired 140 Hz signal from this output signal of the low-pass filter, the above-mentioned French Patent Specification proposes to apply this output signal to a PCM coder to which sampling pulses are applied which occur with a frequency of 4 kHz. The digital signal produced at the output of this PCM coder is thereafter applied to a digital bandpass filter which, albeit in digital form, produces the desired 140 Hz signal. This digital 140 Hz signal is thereafter also applied to a digital comparison circuit wherein the level of this digital 140 Hz signal is compared with a reference level. As the digital output signal of the PCM coder does not only comprise a digital version of the desired 140 Hz signal but also digital versions of the signalling signals, the digital bandpass filter must be a narrow-band filter. This implies that a large number of arithmetical operations must be performed in this filter to obtain one output code word.
The use of the level control arrangement described in the above-mentioned French Patent Specification is only economically justified if the PCM coder can be utilized in time-division multiplex for each one out of a greater number of FDM signals, for example, for 50 primary FDM groups. To enable the use of a PCM coder for such a greater number of primary groups, extremely high demands must be made on the components from which this coder is implemented, as the conversion rate must be very high. The use of one single PCM coder for several mutually independent signals results practice always in cross-talk.